Field of the Invention
The present invention relates in general to methods of fabrication of structures starting from rods of materials by wet chemical etching. Particularly, the present invention relates to methods of fabrication of structures starting from material rods of millimetric or sub-millimetric size. Embodiments of the present invention relates to the fabrication of nanostructures, i.e. structures having al least one feature size in the nanometric range. Embodiments of the present invention relates in particular to the field of nanoprobes, particularly—although not limitatively—optical fiber probes (hereinafter also referred to as optical nanoprobes).
Overview of the Related Art
Optical fiber probes with small size tips (nanotips) have attracted much interest in the areas of biosensors and Scanning Near-field Optical Microscopy (SNOM)
SNOM is a promising imaging technique overcoming diffraction limits. The probe used by SNOM works in close proximity to the surface and images the sample point by point with a resolution that cannot be achieved by classical optical microscopy. Since the first demonstration of SNOM in 1984 (Pohl D. W. et al, 1984 “Optical stethoscopy: Image recording with resolution λ/20”, Appl. Phys. Lett. 44, 651, 1984), this technique has been applied in various areas.
The resolution of SNOM strongly depends on the quality of the optical fiber probe. In many applications the scanning probe, a nanostructure with sub-wavelength aperture at the apex of a metal-coated tapered glass fiber, is the most delicate component.
Desirable properties of such probes are high transmission, obtained by large cone angles, a well defined circular aperture, no light leaking through pinholes in the metal coating, and a high optical damage threshold.
In general, the smaller the probe tip diameter, the better the achievable spatial resolution. However, the bigger the tip angle, the higher the transmission efficiency. Therefore, a good tip is characterized by a high optical transmission, a small apex diameter and a large cone angle. Theoretical calculations and experiments have shown that optical fiber probes with cone angles ranging from 30° to 50° can simultaneously attain high resolution and high transmission efficiency.
The improvement in the fabrication technique of the optical fiber probes also facilitates obtaining probes suitable for optical fiber biochemical sensing. An advantage of optical fiber sensors is the small size of the optical fibers, which enables intracellular sensing of physiological and biological processes in the nanoenvironment.
To perform true non-invasive intracellular analysis, the sensor must be about 100 times smaller than the analyzed cell. The first submicron optical fiber sensor developed by Kopelman and co-workers in 1992 was for pH measurement using fluorescein as a pH indicator (Tan W., Shi Z-Y, Smith S., Birnbaum D. and Kopelman R., “Submicrometer intracellular chemical optical fiber sensors”, Science 258, 778-781, 1992). From then on, more and more miniaturized sensors have been developed for single-cell analysis.
Further increase in the sensitivity of such structures can be obtained by Surface Enhanced Raman Scattering (SERS) or, to obtain better spatial resolution, Tip Enhanced Raman Scattering (TERS) measurements. For SERS, a relatively rough surface of the tip is preferred, so that the metal nanoparticles which produce the enhancement effect can grow on the surface defect sites. On the contrary, for TERS a smooth surface is ideal, in order to concentrate the effect on the metal clad tip.
Most of optical fiber probes are based on tapered optical fibers.
Several methods have been proposed to prepare the tapered glass core necessary for these probes, but essentially all the proposed methods can be divided in two classes: one class includes heating and pulling methods, the other class includes methods based on chemical etching.
In heating and pulling (described e.g. in Valaskovic G. A., Holton M. and Morrison G. H. “Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes”, Appl. Opt. 34, 1215-1234, 1995), the optical fiber is heated by a CO2 laser or a filament and then pulled apart with controlled force, thus producing commonly smooth and long tips. However, it is hard to precisely control the tip diameter and improve the transmission efficiency. Moreover, this technique requires expensive equipment and complicated manipulation.
Chemical etching methods are based on etching glass fibers at the meniscus between hydrofluoric acid and air or an organic overlayer. In comparison to heating and pulling methods, chemical etching allows the production of fiber tips with a shorter cone and a larger cone angle, thereby providing higher optical throughput.
Among all chemical etching methods, static etching, owing to its ease of operation, has been widely employed. Static etching is described in U.S. Pat. No. 4,469,554. A liquid layer is located on top of the etchant (hydrofluoric acid). The liquid in the liquid layer is less dense than the etchant, so that it floats on the etchant. The glass fiber is etched at the meniscus between hydrofluoric acid and the liquid layer. A taper is formed due to a decreasing meniscus height as the fiber diameter is reduced by the etchant. U.S. Pat. No. 4,469,554 also mentions that generally the etchant should etch the material being etched at a reasonable rate and the etch should operate approximately isotropically, although rotation of the material around the cylindrical axis might increase the isotropic nature of the process.